1. Field of the Invention
This invention relates to the field of restart circuitry for a voltage controlled oscillator (VCO) with zero phase compensation.
2. Background Art
Information in computer systems is often transmitted or exchanged in bit stream format. A bit stream consists of a series of logical "1's" or "0's" presented in serial fashion. To accurately decode a serial bit stream, bit frames or bit windows, also known as data recovery windows and decode windows, must be accurately defined. One to one correspondence exists between each bit and each decode window. If the decode window is too large, more than one bit of information may be contained within the window and one or all bits may be lost. If the decode window is too small, no detectable information will result. Further, loss of bit information at point locations may lead to error propagation through the decoding process.
For a variety of reasons, data bit streams may include a degree of "jitter" which tends to force a data bit near a decode window boundary. Jitter is often due to phase errors.
Many integrated circuit applications utilize a voltage controlled oscillator (VCO) to provide timing signals to a data recovery circuit. In some applications, the VCO is used as part of a phased lock look (PLL) as, for example, in connection with a data synchronizer. A VCO is a timing device that provides an output of a certain frequency. The nominal operating frequency of a VCO is referred to as its "center frequency". The VCO can be made responsive to a control voltage V.sub.C so that the output of the VCO can be modified to be greater than or less than the center frequency as required.
A PLL consists of a phase detector, amplifier, filter and VCO. The phase detector compares two input frequencies to generate an output that is a measure of their phase difference. One of the input frequencies is a source signal and the other is the output of the VCO. If the input signals differ in frequency, the phase detector provides a periodic output at the difference frequency. This phase error signal is then filtered and amplified to generate a control voltage V.sub.c that causes the frequency of the VCO to change in the direction of the frequency of the source signal. Eventually, the feedback loop causes the VCO to "lock" to the frequency of the source signal and maintain a fixed phase relationship with the source signal.
The phase comparator circuitry is sensitive to the relative timing of edges between the source signal and the VCO output. The phase comparator generates a lag or lead signal depending on whether the transitions of the VCO output occur before or after the transitions of the source signal. The width of the pulses is equal to the time between the edges of the two input signals to the phase comparator.
In a data recovery system, when data is provided to the system, the VCO is restarted so that the data recovery system can "lock onto" the phase and/or frequency of the data. The VCO output is a locally generated frequency matching the frequency of the input signal, thus providing a clean replica of the input signal frequency. This can compensate for noise that may be present in the input signal. By adjusting the VCO control voltage, the VCO frequency matches the data frequency.
The output of the VCO is provided to a phase detector along with the input signal. The phase detector, driven by the output of the VCO can lock onto the phase of the input signal.
In some data recovery systems, a technique to restart the VCO in the same phase with the incoming data is employed to minimize initial phase error to reduce the transient VCO control voltage and frequency excursion. This is to minimize the frequency and/or phase acquisition period.
In prior art, however, this restart circuitry does not include a compensation of itself, thus, there exists a residual initial phase error. At low data rates (VCO frequency) with relatively large period (in comparison to the residual error), the prior art system shows small control voltage and frequency excursion. But as the incoming data rate increases, so does the voltage excursion and frequency excursion of the VCO, both in absolute terms as well as percentage of the final value.
At high VCO frequency and speeds near the limits of the circuit, the residual error becomes significant. The transient caused by this residual error approaches the worst case possible as though there were no restart circuit. The voltage and frequency excursion of the VCO under this condition is comparable to a VCO which has no zero-phase restart circuitry.